In charged particle beam apparatus including electron microscopes such as a Scanning Electron Microscope (SEM) and a Transmission Electron Microscope (TEM), lenses making use of electrical or magnetic fields are necessarily employed to converge a charged particle beam. In electrostatic or magnetic lenses, various types of aberrations occur inevitably. Consequently, even when trying to narrow down a charged particle beam with a high ratio of demagnification, if there is a large aberration, it is impossible to decrease the diameter of a spot and it is impossible to improve the precision of observing microstructures and measuring dimensions.
In charged particle beam apparatus, the introduction of an aberration corrector is promoted for the purpose of improving resolution. The aberration corrector is normally configured with multipole lenses installed in multiple stages and generates an electrical or magnetic field inside the multipole lenses, thereby removing an aberration included in a charged particle beam passing through an opening inside the lenses.
As regards the aberration corrector, there is one that uses four stages of multipole lenses, for example, as disclosed in Non Patent Literature 1.
In charged particle beam apparatus, there is an aberration caused by the fact that an accelerating voltage is not completely constant and has a range of energy. Those typical of this aberration are a chromatic aberration and a magnification chromatic aberration. In the present specification, an amount of dispersion (blur) caused by the magnification chromatic aberration is particularly referred to as a chromatic dispersion aberration. In the aberration corrector of Non Patent Literature 1, a technique for correcting the chromatic aberration is disclosed. In Non Patent Literature 1, however, there is no disclosure about measurement of a chromatic third-order aperture aberration or chromatic third-order aperture aberration and a technique for correcting a chromatic third-order aperture aberration when a tilt observation technique is applied.
An issue concerning the chromatic third-order aperture aberration during aberration correction is disclosed in Non Patent Literature 2. Non Patent Literature 2 discloses conditions under which resolution is limited by an increase in the chromatic third-order aperture aberration and a fifth-order spherical aberration as a result of aberration correction and, it is disclosed that, as a countermeasure, an effective length between the aberration corrector and an objective lens should be shortened. In Non Patent Literature 2, however, a method of measuring an actual chromatic third-order aperture aberration is not disclosed and a method of counterbalancing a chromatic third-order aperture aberration and some other aberration during tilt observation is not disclosed as well.
As a tilt observation technique taking account of aberration correction for charged particle beam apparatus, there exists, for example, a technique that is disclosed in patent literature 1. This technique is disclosed as a technique for improving resolution by performing aberration correction in tilt observation. However, there is no disclosure about measurement of a chromatic third-order aperture aberration and a technique for correcting a chromatic third-order aperture aberration during tilt observation.
Moreover, as a technique for detecting a chromatic dispersion aberration of charged particle beam apparatus and correcting it, there exists, for example, a technique that is disclosed in patent literature 2. This technique is adapted to correct a beam displacement in a situation when the beam becomes off axis with an accelerating voltage being changed from a current value. However, there is no disclosure about an aberration corrector and a tilt observation technique by means of the aberration corrector.